Generally, for detecting the level of a material stored in a vessel vibrating gauges are employed which operate on the basis of oscillatory portions as for example coaxial tube oscillators or tuning forks. For detecting whether a predetermined switching level is reached or not, these systems analyze the attenuation and/or the frequency shift of the resonance frequency of the electromechanical oscillation system once the oscillating portion of the vibration level gauge immerses into the filling material.
For example, a vibration level gauge is known from DE 33 48 119 C2 which is adapted to detect a predetermined filling level and which comprises oscillatory portions as for example an oscillating rod probe or tuning fork protruding into a vessel and being excited by means of an electrical actuation unit. Since the rod probe or the tuning fork is part of a feedback circuit, the frequency of the rod probe or tuning fork depends on the filling level in the vessel. Hence, by amplifying, and due to the feedback of the electrically detected mechanical oscillation signal onto the actuation input, an oscillatory system comes into being which is generally know as an oscillator.
In contrast to said method, which uses an oscillatory mechanical portion as a part of an electromechanical oscillator, it is known from DE102 03 461 to detect a predetermined filling level by externally actuating an oscillatory system in its resonance frequency by means of an electric oscillator, measuring of at least one characteristic of the systems response on said actuation and analyzing these characteristics. However, this method is only adapted to detect a filling level if the system is operated in its resonance frequency or at least in a frequency band which is very close to said resonance frequency.
However, since a failure in the functionality of such a vibration level gauge may cause serious damages as, for example, overflowing of a vessel or dry running of a pump, it is known to supply such a vibrating level gauge with various internal testing options in order to detect a failure in the operation and to generate an appropriate notice of failure.
Therefore, it is known, for example, from DE 198 40 796 D1, to adjust the gain of a feedback amplifier for the purposes of a self-test whereby it is possible to enforce a notice that a predetermined filling level of a vessel is reached on a trial basis.
Another test method is disclosed in DE 100 23 305 A1, wherein the capacity or inductivity between the feed cables leading to a piezoelectric element is detected during the vibrational agitation. Once the detected value of the capacity or inductivity differs from a predetermined set point, a notice of failure occurs.
Finally, DE 44 02 234 describes to temporarily break the connection between an amplifier and an actuation transducer and to analyze system failures from a phase shift detected due to the break of the connection.
However, the above illustrated methods suffer from the disadvantage that the result of the self-test is either not totally independent from the filling level prevailing at the moment of the self-test or since not the whole actuation element required for the vibrational agitation is involved in the self-test. Therefore, the reliability of said self-diagnosis is limited. Moreover, the above illustrated methods are merely adapted to detect and signal a total failure of a gauge whereas failures in sub-assemblies of the gauge pass undetected. Furthermore, the results of these diagnosing processes are derived from only one physical characteristic of the gauge which is normally at the same time the measured quantity wherefrom a notice may be derived, that a predetermined filling level is reached.